Method for glossing articles made of aluminum and particularly pure aluminum and itsalloys



United States Patent Germany No Drawing. Filed Jan. 12, 1961, Ser. No. 82,180 Claims priority, applicatiiglsgzrmany, Jan. 13, 1960,

Claims. 331. 15621) The present invention relates to a composition and method for glossing aluminum. More particularly, the present invention is concerned with the glossing of aluminum of relatively low purity and with the glossing of aluminum alloys containing such low-purity aluminum and generally being of the type of aluminum alloys to which gloss or luster can be imparted followed by electrolytic oxidation.

Treating solutions have been described for the purpose of forming a glossy surface on aluminum and aluminum alloy bodies. Among these treating solutions are those which consist of aqueous solutions containing hydrogen, ammonium, nitrate and fluoride. ions, whereby it is desired to adjust the quantative relationship between fluoride and ammonium ions so as to favor the formation of NH HF It has also been suggested to incorporate in such solutions a relatively small amount of lead ions.

Solutions of the type described above particularly containing 10 to 15, preferably 13, percent by weight of nitric acid, and 10 to 20, preferably 16, percent by weight of ammonium hydrogenfluoride besides having a content of between 0.05 and 0.5 gram of lead per liter, especially give very good results in cases where the aluminum surface which is to be treated consists of high-purity aluminum or suitable alloys thereof. However, the results were less favorable when it was attempted to use this type of solutions for imparting high-gloss to aluminum or aluminum alloys in which the aluminum was of lesser purity.

It has been attempted, and frequently is desirable, to'

impart high gloss to bodies formed of aluminum of lesser purity or of aluminum alloys suitable for glossing followed by anodising treatment in which the aluminum is of lesser purity. Particularly, great interest has been shown in the glossing of aluminum having a purity of 99.95% and less, especially a purity of between 99.8 and 99.95%. It was found that the prior art glossing or treating solutions of the type described above, without or with the addition of gum arabic or dextrinwith these substances in a concentration of 0.5 to 2, preferably 1%-- as a hydrophilic colloid such as an inhibitor, did not impart the desired high gloss to such bodies containing aluminum of a purity of 99.95% and less.

It is therefore an object of the present invention to overcome the difliculties encountered in attempts to impart high gloss to aluminum or aluminum alloy bodies of the type suitable for glossing followed by electrolytic oxidation wherein the aluminum is of lesser purity, such as a purity of no more than 99.95

It is another object of the present invention to provide a composition and a method for imparting high gloss to aluminum and aluminum alloy bodies in which the purity of the aluminum is between 99.8 and 99.95%, or even below 99.8%.

It is a further object of the present inventon to provide a method for regenerating treating solution which slightly deteriorates in its glossing effect in the treatment of aluminum surfaces.

Other objects and advantages of the present invention will become apparent from a further reading of the description and of the appended claims.

With the above and other objects in view, the present invention contemplates a treating solution adapted to form a glossy surface on a body of a substance belonging to the group consisting of aluminum having a purity of up to 99.95% and its alloys adapted to be lustered followed by electrolytic oxidation, the treating solution consisting of an aqueous solution containing between 6.4 and 10.2% by weight of HNO between 5.4 and 8.8% by weight of NH,,HF between 0.12 and 0.35 gram of lead ions per liter, and between 0.4 and 8.8% by weight of a hydrophilic colloid.

Surprisingly, it has been found according to the present invention that a high gloss can be formed on aluminum bodies of lesser purity by treating such aluminum or glossable aluminum alloys with treating solutions which contain nitric acid, ammonium hydrofluoride, lead and a hydrophilic colloid from a group consisting of gum arabic, dextrin, agar-agar as an inhibitor, provided that the quantities and proportions of the individual constituents of the treating solution are strictly controlled and maintained within the ranges described above.

The present invention, however, contemplates in a method of forming a glossy surface on a body of a substance belonging to the group consisting of aluminum having a purity of up to 99.95% and of its alloys adapted to be lustered followed by electrolytic oxidation, the step of treating the surface at a temperature of between about 65 and C. for between about 40 and 60 seconds with an aqueous solution containing the equivalent of between and 160 cc. 54% nitric acid, the equivalent of between 25 and 40 cc. 70% hydrofluoric acid and water to form one liter of an aqueous solution of nitric acid and hydrofluoric acid wherein there is added to the thus formed solution between 40 and 70 grams lead-free ammoniumfluoride, an amount of lead equivalent to between 0.2 and 0.6 gram lead nitrate and being present in the form of a water soluble lead salt, and between 5 and 100 grams gum arabic.

Especially useful results are obtainable with an aqueous solution which may be produced by mixing cc. of 54% nitric acid and 33 cc. of 70% hydrofluoric acid with suflicient water to form one liter of an aqueous solution of nitric acid and hydrofluoric acid; and to add to the thus formed solution 50 grams ammonium fluoride, 0.3 gram lead nitrate and 30 grams gum'arabic. In this manner, 1,060 cc. of an aqueous solution having a specific weight of 1.074 are formed.

The treating solution according to the present invention is an aqueous solution containing between 6.4 and 10.2% by weight, preferably about 8.5% HNO between 5.4 and 8.8% by Weight, preferably about 6.8% NH HF between 0.12 and 0.35 gram per liter, preferably about 0.18 gram per liter lead ions; and between 0.4 and 8.8% by Weight of hydrophilic organic colloid, preferably about 2.6% by Weight of gum arabic.

Instead of lead nitrate, other water soluble lead salts, for instance, lead acetate, may be used, provided that the concentration of lead ions in the treating solution is maintained at a level corresponding to between about 0.2 and 0.6 gram lead nitrate per liter, or to between 0.12 and 0.35 gram per liter of lead ions.

Instead of gum arabic, also other viscosity-increasing hydrophilic colloids such as dextrin or agar-agar may be used.

The ammonium fluoride which is used in producing the treating solution of the present invention must be free of lead.

Thus, broadly, the treating solution of the present invention may be formed by adding water to between 100 and cc. of 54% nitric acid and between 25 and 40 cc. 70% hydrofluoric acid so as to form one liter of solution, and to introduce into the thus formed solution between 40 and 70 grams of lead-free ammonium fluoride, between 0.2 and 0.6 gram lead nitrate (or of another water soluble lead salt containing an equivalent amount of lead), and between and 100 grams of gum arabic, dextrin, agar-agar or the like.

Treatment of the aluminum surface of relatively low purity with the above described treating solution is carried out for a period of between about 40 and 60 seconds depending on the condition of the aluminum surface. When using freshly prepared treating solution, the treating temperature is preferably maintained between 65 and 75 C., in case the treating solution has aged, it is preferred to carry out the treatment at somewhat higher temperatures, up to about 85 C.

Surprisingly, it has been found that application of the treating solution described above will allow to obtain on aluminum of lesser purity, particularly on aluminum having a purity of between 99.8 and 99.95% (as well as on alloys thereof) a degree of gloss or reflective values which come close to those obtainable with aluminum surfaces of highest purity and which are higher than the gloss that could be obtained heretofore on aluminum surfaces of lesser purity. Moreover, the degree of gloss obtained with treating solutions according to the present invention on a given aluminum surface is reproducibly constant to an extent not attainable with the prior art treating solution containing substantially the same constituents but not in the strictly controlled quantative relationships described above.

During use of the treating solution of the present invention. the same becomes spent more and more and slowly loses its effectiveness, if a regeneration of the solution is not carried out. It is desirable to preserve the high effectiveness of the solution for imparting to aluminum surfaces the same high degree of gloss which it was initially capable of producing. The gradual exhaustion of the treating solution depends primarily on the square area of aluminum surface which has been treated with a given quantity of solution, however, the surface condition of the aluminum body also affects the exhaustion of the solution.

Generally, it has been found that best results are obtained by treating between about 0.25 and 0.30 m? per liter of freshly prepared solution.

However, it is not necessary and in fact inadvisable, to discard the thus once used treating solution.

After having treated between about 0.25 and 0.30 m? with one liter of treating solution and thus slightly spent the same, it is preferred, according to the present invention to add per liter of this partially spent solution between and cc., preferably 15 cc. of 70% hydrofluoric acid and between 0.05 and 0.1 gram, preferably 0.08 gram of lead nitrate dissolved in a small quantity of water. Of course, the lead nitrate may be replaced by an equivalent amount of another water soluble lead salt. The thus regenerated solution may now be used for glossing an additional 0.15 to 0.20 m? of aluminum surface per liter of solution. Thereafter, the again partially spent solution may be again regenerated as described above, and regeneration may be repeated after each additional treatment of between about 0.15 and 0.20 m? of aluminum surface, until per liter of solution a total surface area of about 2 111. has been treated.

According to the present invention, it is not only possible to obtain a high gloss on surfaces containing aluminum of relatively low purity, but the present method and solution shows also additional advantages.

Surprisingly it has been found that the treatment solution according to the present invention allows incorporation of relatively large quantities of gum arabic. To increase the proportion of gum arabic is desirable, however, up to now only relatively small quantities of gum arabic could be incorporated in glossing solutions due to the fact that increasing the concentration of gum arabic resulted in the formation of an uneven blue haze on the glossy surface. On the other hand, particularly with respect to aluminum surfaces of relatively low purity, it is most desirable to increase the proportion of gum arabic in order to counteract the etching effect of the treating bath. This is now possible.

Furthermore, the capacity of the treating solution according to the present invention, i.e., the square area which can be treated with a given quantity of solution, is about twice as great than that of qualitatively more or less similar solutions which however are not limited to the specific quantitive relationships of the present invention. Thus, up to now it was only possible to treat about 1 m? of aluminum surface per liter of treating solution, while according to the present invention and as described above, about 2 m? of relatively low purity aluminum surface can be successfully treated with one liter of solution.

In addition, the close control of the lead ion content of the initial solution and of the regenerated solution according to the present invention will assure optimum lustering effect throughout the entire useful life span of the treating solution, i.e., until the solution which has been regenerated several times has been used for about 2 in. of aluminum surface per liter of solution. This again is particularly important in the glossing of aluminum which is not of highest purity.

The following examples are given as illustrative only of the present invention, without being intended to limit the invention to the specific details of the examples.

The examples describe the treatment in accordance with the present invention of 5 aluminum and aluminum alloy sheets of different composition and compare the results with results obtained by treating identical sheets with conventional treating solution.

The composition of the aluminum sheets was as follows:

Examplel A1 99.8% pure+0.2% Mg. Example 2 A1 99.9% pure.

Example 3 A1 99.9% pure+0.5% Mg. Example4 A1 99.9% pure+1% Mg. Example 5 Al 99.99% pure.

Treatment A was carried out in accordance with the present invention by treating the aluminum sheets after conventional de-fatting, for 40 seconds at 65 C. in a bath which was formed by making up cc. of 54% nitric acid and 33 cc. of 70% hydrofluoric acid with water so as to obtain one liter of solution and by adding to one liter of the thus formed solution 50 grams of lead-free ammonium fluoride, 0.3 gram lead nitrate and 30 grams gum arabic. The thus treated aluminum sheets were then rinsed with water, immersed for two minutes into 30% nitric acid at room temperature, again rinsed with water and then in conventional manner supplied with an anodically produced protective layer having a thickness of 5 microns.

Treatment 13" was carried out according to the prior art, by conventionally de-fatting the aluminum sheets and thereafter treating the same for 20 seconds at 50 C. in a treatment solution which contained 16% by weight of ammonium hydrogenfiuoride, 13% by weight of nitric acid, 0.18 gram per liter of lead and 10 grams per liter of gum arabic. The further treatment of the aluminum sheets after removal from the treating bath, was carried out in exactly the same manner as described under The following results were obtained:

A Bu

It will be seen that a considerably higher gloss was obtained according to the present invention. In fact, the gloss thus obtained on aluminum sheets of relatively low purity came close to the gloss obtainable according to the prior art in the treatment of high purity aluminum (Example Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

1. A treating solution to form a glossy surface on a body of a substance belonging to the group consisting of aluminum having a purity of up to 99.95% and of alloys of said aluminum adapted to be lustered followed by electrolytic oxidation, said treating solution consisting of an aqueous solution containing the equivalent of between 100 and 160 cc. 54% nitric acid; the equivalent of between 25 and 40 cc. 70% hydrofluoric acid; and water to form one liter of an aqueous solution of nitric acid and hydrofluoric acid wherein there is added to the thus formed solution between 40 and 70 grams lead-free ammonium fluoride; an amount of lead equivalent to between 0.2 and 0.6 gram lead nitrate and being present in the form of a water soluble lead salt; and between 5 and 100 grams gum arabic.

2. A treating solution adapted to form a glossy surface on a body of a substance belonging to the group consisting of aluminum having a purity of between 99.8 and 99.95 and ofalloys of said aluminum adapted to be lustered followed by electrolytic oxidation, said treating solution consisting of an aqueous solution containing the equivalent of between 100 and 160 cc. 54% nitric acid; the equivalent of between 25 and 40 cc. 70% hydrofluoric acid; and water to form one liter of an aqueous solution of nitric acid and hydrofluoric acid wherein there is added to the thus formed solution between 40 and 70 grams lead-free ammonium fluoride; an amount of lead equiva-' lent to between 0.2 and 0.6 gram lead nitrate and being present in the form of a water soluble lead salt; and between 5 and 100 grams gum arabic.

3. A treating solution adapted to form a glossy surface on a body of a substance belonging to the group consisting of aluminum having a purity of up to 99.95% and of alloys of said aluminum adapted to be lustered followed by electrolytic oxidation, said treating solution consisting of an aqueous solution containing the equivalent of 135 cc. 54% nitric acid; the equivalent of 33 cc. 70% hydrofluoric acid; and water to form one liter of an aqueous solution of nitric acid and hydrofluoric acid wherein there is added to the thus formed solution 50 grams lead-free ammonium fluoride; and 30 grams gum arabic.

4. In a method of forming a glossy surface on a body of a substance belonging to the group consisting of aluminum having a purity of up to 99.95% and of alloys of said aluminum adapted to be lustered followed by electrolytic oxidation, the step of treating said surface at a temperature of between about 65 and 85 C. for between about 40 and 60 seconds with an aqueous solution containing the equivalent of between 100 and 160 cc. 54% nitric acid; the equivalent of between 25 and 40 cc. 70% hydrofluoric acid; and water to form one liter of an aqueous solution of nitric acid and hydrofluoric acid wherein there is added to the thus formed solution between 40 and 70 grams lead-free ammonium fluoride; an amount of lead equivalent to between 0.2 and 0.6 gram lead nitrate and b ing pr s nt in th form of a water soluble lead salt; and between 5 and grams gum arabic.

5. A method of regenenating partially spent treating solution according to claim 4, comprising the step of adding to said partially spent treating solution, per liter thereof: between 10 and 25 cc. 70% hydrofluoric acid,

and between 0.05 and 0.1 gram of lead nitrate in the form of a concentrated aqueous solution.

6. A method of regenerating partially spent treating solution according to claim 4, comprising the step of adding to said partially spent treating solution, per liter thereof: about 15 cc. 70% hydrofluoric acid, and about 0.08 gram of lead nitrate in the form of a concentrated aqueous solution.

7. A method according to claim 4, wherein after treating between 0.25 and 0.30 m. of said surface per liter of said aqueous solution, between 10 and 25 cc. 70% hydrofluoric acid and a concentrated aqueous solution of a water soluble lead salt in a quantity equivalent to between 0.05 and 0.1 gram lead nitrate are added to said solution so as to regenerate the same.

8. A method according to claim 4, wherein after first treating between 0.25 and 0.30 m. of said surface per liter of said aqueous solution, between 10 and 25 cc. 70% hydrofluoric acid and a concentrated aqueous solution of a water soluble lead salt in a quantity equivalent to between 0.05 and 0.1 gram lead nitrate are added to said solution so as to regenerate the same; the thus regenerated solution then being used for treatment of an additional between 0.15 and 0.20 m. of said surface, thereby being again partially spent; and the thus again partially spent solution is again regenerated by adding said hydrofluoric acid and said soluble lead salt thereto, and whereby said additional treatment and subsequent regeneration of said solution may be repeated until a total area of about 2 m. of said surface has been treated per liter of said solution.

9. A treating solution adapted to form a glossy surface on a body of a substance belonging to the group consisting of aluminum having a purity of up to 99.95 and of alloys of said aluminum adapted to be lustered followed by electrolytic oxidation, said treating'solution consisting of an aqueous solution containing between 6.4 and 10.2% weight of HNO between 5.4 and 8.8% by weight of NH.,HF between 0.12 and 0.35 gram of lead ions per liter; and between 0.4 and 8.8% by Weight of a hydrophilic colloid.

10. A treating solution adapted to form a glossy surface on a body of a substance belonging to the group consisting of aluminum having a purity of up to 99.95% and of alloys of said aluminum adapted to be lustered followed by electrolytic oxidation, said treating solution consisting of an aqueous solution containing about 8.5% by weight of HNO about 6.8% by weight of NH HF,; about 0.18 gram of lead ions per liter; and about 2.6% by weight of a hydrophilic colloid.

References Cited in the file of this patent UNITED STATES PATENTS 2,625,468 France et al Jan. 13, 1953 2,719,781 Hesch Oct.'4, 1955 2,746,849 Helling et a1 May 22, 1956 2,756,131 Neunzig July 24, 1956 2,828,193 Walter Mar. 25, 1958 2,967,136 Cybriwsky et al Ian. 3, 1961 FOREIGN PATENTS 1,082,346 France June 16, 1954 806,608 Great Britain Dec. 31, 1958 OTHER REFERENCES Hackhs Chemical Dictionary, Grant, 3rd Edition, 1944, p. 348. 

1. A TREATING SOLUTION TO FORM A GLOSSY SURFACE ON A BODY OF A SUBSTANCE BELONGING TO THE GROUP CONSISTING OF ALUMINUM HAVING A PURITY OF UP TO 99.95% AND OF ALLOYS OF SAID ALUMINUM ADAPTED TO BE LUSTERED FOLLOWED BY ELECTROLYTIC OXIDATION, SAID TREATING SOLUTION CONSISTING OF AN AQUEOUS SOLUTION CONTAINING THE EQUIVALENT OF BETWEEN 100 AND 160 CC. 54% NITRIC ACID; THE EQUIVALENT OF BETWEEN 25 AND 40 CC. 70% HYDROFLUORIC ACID; AND WATER TO FORM ONE LITER OF AN AQUEOUS SOLUTION OF NITRIC ACID AND HYDROFLUORIC ACID WHEREIN THERE IS ADDED TO THE THUS FORMED SOLUTION BETWEEN 40 TO 70 GRAMS LEAD-FREE AMMONIUM FLUORIDE; AN AMOUNT OF LEAD EQUIVALENT TO BETWEEN 0.2 AND 0.6 GRAM LEAD NITRATE AND BEING PRESENT IN THE FORM OF A WATER SOLUBLE LEAD SALT; AND BETWEEN 5 AND 100 GRAMS GUM ARABIC.
 4. IN A METHOD OF FORMING A GLOSSY SURFACE ON A BODY OF A SUBSTANCE BELONGING TO THE GROUP CONSISTING OF ALUMINUM HAVING A PURITY OF UP TO 99.95% AND OF ALLOYS OF SAID ALUMINUM ADAPTED TO BE LUSTERED FOLLOWED BY ELECTROLYTIC OXIDATION, THE STEP OF TREATING SAID SURFACE AT A TEMPERATURE OF BETWEEN ABOUT 65 AND 85*C. FOR BETWEEN ABOUT 40 AND 60 SECONDS WITH AN AQUEOUS SOLUTION CONTAINING THE EQUIVALENT OF BETWEEN 100 AND 160 CC. 54% NITRIC ACID; THE EQUIVALENT OF BETWEEN 25 AND 40 CC. 70% HYDROFLUORIC ACID; AND WATER TO FORM ONE LITER OF AN AQUEOUS SOLUTION OF NITRIC ACID AND HYDROFLUORIC ACID WHEREIN THERE IS ADDED TO THE THUS FORMED SOLUTION BETWEEN 40 AND 70 GRAMS LEAD-FREE AMMONIUM FLUORIDE; AN AMOUNT OF LEAD EQUIVALENT TO BETWEEN 0.2 AND 0.6 GRAM LEAD NITRATE AND BEING PRESENT IN THE FORM OF A WATER SOLUBLE LEAD SALT; AND BETWEEN 5 AND 100 GRAMS GUM ARABIC. 